A dysrhythmia is an abnormal heart beat pattern. One example of a dysrhythmia is a bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Other examples of dysrhythmias include tachyarrhythmias wherein the heart beats at an abnormally fast rate. With atrial tachycardia, the atria of the heart beat abnormally fast. With ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a tachycardia is typically not fatal. However, some tachycardias, particularly ventricular tachycardia, can trigger ventricular fibrillation wherein the heart beats chaotically such that there is little or no net flow of blood from the heart to the brain and other organs. Ventricular tachycardia, if not terminated, is fatal. Hence, it is highly desirable to prevent or terminate dysrhythmias, particularly ventricular tachycardias.
One technique for preventing or terminating dysrhythmias is to overdrive pace the heart wherein an implantable cardiac stimulation device, such as a pacemaker or implantable cardioverter defibrillator (ICD), applies electrical pacing pulses to the heart at a rate somewhat faster than the intrinsic heart rate of the patient. For bradycardia, the cardiac stimulation device may be programmed to artificially pace the heart at a rate of 60 to 80 pulses per minute (ppm) to thereby prevent the heart from beating too slow and to eliminate any long pauses between heart beats. To prevent tachyarrhythmias from occurring, the cardiac stimulation device artificially paces the heart at a rate of at least five to ten pulses per minute faster than the intrinsic tachyarrhythmia heart rate of the patient. In other words, a slight artificial tachycardia is induced and maintained in an effort to prevent an actual tachycardia from arising. If an actual tachycardia occurs, such as a supraventricular tachycardia (SVT) wherein the heart may begin beating suddenly at 150 beats per minute or more, the cardiac stimulation device senses tachycardia and immediately begins pacing at a rate of at least five to ten pulses per minute (ppm) faster than the tachycardia and then slowly decreases the pacing rate in an effort to slowly reduce the heart rate back to a normal resting rate, thereby terminating the tachycardia.
It is believed that overdrive pacing is effective for at least some patients for preventing or terminating the onset of an actual tachycardia for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patents, however, additional portions of the heart also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods which, in turn, can trigger a tachycardia. By overdrive pacing the heart at a uniform rate, the likelihood of the occurrence of ectopic pulses is reduced and the refractory periods within the heart tissue are rendered uniform and periodic. Thus, the dispersion of refractory periods is reduced and tachycardias triggered thereby are substantially avoided. If a tachycardia nevertheless occurs, overdrive pacing at a rate faster than a tachycardia helps to eliminate ectopic pulses and reduce refractory period dispersion, and thereby helps to terminate the tachycardia.
Thus, it is desirable within patients prone to tachyarrhythmias to ensure that most beats of the heart are paced beats, as any unpaced beats may be ectopic beats. A high percentage of paced beats can be achieved simply by establishing a high overdrive pacing rate. However, a high overdrive pacing rate has disadvantages as well. For example, a high overdrive pacing rate may be unpleasant to the patent, particularly if the artificially-induced heart rate is relatively high in comparison with the heart rate that would otherwise normally occur. A high heart rate may also cause possible damage to the heart or may possibly trigger more serious dysrhythmias, such as a ventricular fibrillation.
A high overdrive pacing rate may be especially problematic in patients suffering from heart failure, particularly if the heart failure is due to an impaired diastolic function. A high overdrive pacing rate may actually exacerbate heart failure in these patients. Also, a high overdrive pacing rate may be a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia. Also, the need to apply overdrive pacing pulses operates to deplete the implantable cardiac stimulation device's power supply, perhaps requiring frequent surgical replacement of the power supply. Typically, the power supply is located within the implantable cardiac stimulation device and thus this requires surgical replacement of the cardiac stimulation device.
Problems associated with overdrive pacing are particular severe for certain aggressive overdrive techniques which trigger an increase in the pacing rate based upon detection of a single intrinsic heart beat. With such techniques, a significant increase in the pacing rate is triggered by detection of a single intrinsic heart beat so as to promptly respond to the occurrence of a high rate tachycardia, such as an SVT. As a result, even in circumstances where a high rate tachycardia has not occurred, the detection of a single intrinsic heart beat can cause a significant increase in the overdrive pacing rate, which may be reduced only gradually. If a second intrinsic heart beat is detected before the overdrive pacing rate has been gradually lowered to a standard overdrive pacing rate, a still further increase in the pacing rate occurs. As can be appreciated, the foregoing can cause the overdrive pacing rate to increase significantly, perhaps to 150 ppm or more, even though a high rate tachycardia has not occurred.
Hence, it would be desirable to provide techniques for overdrive pacing which reduce the average overdrive pacing rate, yet still attain a sufficiently high rate to significantly reduce the likelihood of a dysrhythmia within the patient or to terminate a dysrhythmia if one nevertheless occurs. In particular, it would be highly desirable to provide overdrive pacing techniques which permit a certain percentage of paced beats (such as 90% or 95%) to be sustained by the cardiac stimulation device so as to enable the overdrive pacing rate to be minimized while still ensuring that most beats of the heart are paced beats. It is to these ends that aspects of the present invention are primarily directed.